Add `upgradeable_version` property

to Contract class

slither/core/declarations/contract.py

@@ -88,6 +88,7 @@ class Contract(SourceMapping):  # pylint: disable=too-many-public-methods
 
         self._is_upgradeable: Optional[bool] = None
         self._is_upgradeable_proxy: Optional[bool] = None
+        self._upgradeable_version: Optional[str] = None
 
         self.is_top_level = False  # heavily used, so no @property
 
@@ -1246,6 +1247,14 @@ class Contract(SourceMapping):  # pylint: disable=too-many-public-methods
                                     return self._is_upgradeable_proxy
         return self._is_upgradeable_proxy
 
+    @property
+    def upgradeable_version(self) -> Optional[str]:
+        return self._upgradeable_version
+
+    @upgradeable_version.setter
+    def upgradeable_version(self, version_name: str):
+        self._upgradeable_version = version_name
+
     # endregion
     ###################################################################################
     ###################################################################################